The present invention relates to high consistency refining of lignocellulosic material, and in particular to refining of wood chips and the like for production of mechanical pulp.
U.S. Pat. No. 5,776,305 discloses a method which achieves significant advances in the trade-off between pulp quality and energy utilization, by preheating the feed chips at a high steam saturation temperature (T) for a short residence time (R), followed by high intensity, high speed disc refining (S). The preheat and refining temperature (T) is above the glass transition temperature of the lignin (i.e., above Tg). The process described in the ""305 patent can be conveniently referred to as xe2x80x9cRTSxe2x80x9d. The disclosure of U.S. Pat. No. 5,776,305, is hereby incorporated by reference.
It is an object of the present invention, to further extend the benefits observed with the RTS process described in U.S. Pat. No. 5,776,305.
Such further benefits have indeed been realized in accordance with the present invention, whereby the combination of low residence time (R) with high saturation pre-heat temperature/pressure (T) followed by high speed disc refining (S), is utilized in both primary and secondary refining stages.
This use of the RTS process in both primary and secondary refining stages, can be accomplished in one embodiment, where the primary refining is performed in a first disc refiner and the secondary refining is performed in a distinct second disc refiner. In a particularly cost-effective second embodiment the primary and secondary refining are performed within a single refining machine having distinct primary and secondary refining zones which are fluidly connected in series. For convenience, the invention in any of these embodiments or variations, will be referred to as the RTS2 system or process.
In laboratory tests of the present invention using spruce furnish, latency chest freeness levels were obtained at 200 ml, with refiner total specific energy requirements of only 1348 kwh/ODMT. Relative to conventional TMP pulp, the process according to the present invention can achieve improved strength properties and lower shive content, with more than 400 kwh/ODMT savings in energy requirements.
A further improvement to the RTS2 system and process may be achieved by careful selection of different refiner plate patterns, temperature, effective residence time, and refiner speed in the second refiner. In some circumstances, it may be preferable for the secondary conditions to be somewhat less severe than in the primary, i.e., a pressure in the range of 65-85 psi, residence time in the range of under 30 seconds, and refiner speed of 1800-2600 rpm in the secondary, versus a pressure range of 75-95 psi, residence time of 15 seconds or less, and speed of 2000-2600 rpm in the primary. In the embodiment wherein the primary and secondary refining zones are in a single machine and the disc rotation speeds will likely be identical, it is most preferred that the preheat residence time and temperature in the secondary, be somewhat less than that in the primary. For example, the secondary refining pressure can be up to about one bar lower than the primary refining pressure.